The present invention relates to an apparatus that detects the occurrence of slack in a cable, and compensates for detected slack by adding preloaded tension in the cable. The apparatus further generates a warning signal when the slack in the cable exceeds a certain extent. More particularly, the detection and compensation apparatus in accordance with the present invention detects and compensates for slack in the door cable when a door operator lifts and lowers a door by selectively winding and unwinding such cable. The present invention also relates to a door motor drive control system that is equipped with such a cable slack detection and compensation device.
A common, conventional commercial door operator system includes a shaft known as a "jack shaft", a drum mounted on the jack shaft, a cable windable on such drum, and a motor drive for turning the shaft and the drum to selectively wind and unwind the cable. The cable is connected to the door so that the motor lifts the door to open the door by rotating the drum in one direction, and lowers the door by rotating the drum in the opposite direction to pay out the cable. In typical commercial door installations, the door is "balanced"through the cable and paying out the cable allows the door to be closed by gravity. Such a commercial door is lowered by gravity and the lowering speed of the door is controlled by the rate at which the cable is unwound from the drum.
It is important that tension remains in the cable as the door is being lowered. As long as tension remains in the cable, the drum properly will pay out the cable without risk that coils of cable wound on the drum will release from the drum prematurely. Loss of tension in the cable will occur if the door encounters an obstruction, most often in the door opening, as the door is lowered.
Obstruction detection is inherent in residential door operators, but is much more difficult to implement in commercial settings. The typical capacitor start motor powered commercial door operator does not sense when an obstruction impedes or prevents lowering of the door, and consequently the operator will continue to unwind the cable from the drum. Thus, if an obstruction is detected in some other way, such as visually by an operator and then the obstruction is removed, the door will fall suddenly because the cable has developed slack due to having been unwound partially or fully from the drum prior to the detection of the obstruction. This situation leads to obvious danger to people and property below the door, and the door itself, due to uncontrolled door acceleration. Even a small degree of slack potentially is dangerous because under slack conditions, windings of cable can leave the drum and thus lead to the same uncontrolled downward acceleration.
Generally, in hoisting apparatus relying upon a cable for lifting a load, it is conventional to detect and provide alarm conditions when the cable is underloaded or overloaded. For example, U.S. Pat. No. 2,636,953 relates to the use of a slack detection system safety switch. According to the inventor of U.S. Pat. No. 2,636,953, as long as the monitored cable is under sufficient tension, a contracting-bias spring will be extended and a cam follower will have ridden down an inclined cam surface. If slack develops in the monitored cable, the bias spring contracts and the cam follower moves up the incline cam surface to change the condition of a switch and thereby turn off the hoist motor.
U.S. Pat. No. 1,432,058 discloses an early door operator. This operator features a compensating beam that is balanced so that excessive strain will overcome spring force which tends to hold the beam in balance and thus tilt the beam to trigger a switch that provides a signal stopping the motor.
U.S. Pat. No. 394,952 represents an early hoisting arrangement, apparently for an elevator. It protects against both underload and overload on the cable by a contact bar that, during normal conditions, is centered between overload and underload switch contacts.
U.S. Pat. No. 5,361,565 also discloses an elevating system that accounts for overload and underload. It shows a winch assembly equipped with an overload/underload protector. If underload occurs, a compression spring is relieved of pressure and expands to trip an underload limit switch.
U.S. Pat. 3,012,520, also relates to a garage door opener. Springs allow the opening and closing of the door until there is a certain amount of predetermined opposing force preventing door movement.
U.S. Pat. Nos. 5,348,373; 5,461,826; 5,477,641; 2,225,003; 3,936,622; and 3,612,487 also have been noted in regard to detecting underload and/or overload.
What is missing in the conventional arrangements such as those discussed above, is a safety device that is capable of preloading additional tension in the cable to eliminate dangerous slack conditions therein. Then, if the device can not tension the cable sufficiently, the device should signal that the cable still remains slack. This protects against slack conditions in a door operator by equipping the operator to stop the motor immediately from further unwinding the cable when such a slack condition warning signal is received.